The traditional design of an anvil for use in an impact wrench includes a round portion that transitions to a square portion. The round portion is received within the impact wrench and acts as a bearing journal. The square portion is received within an impact socket. The transition from the round cross section to the square cross section inherently creates sharp corners or small radii within the transition.
These sharp corners or small radii may create some inefficiencies in the design. Initially there is minimal clearance between the square portion of the anvil and the impact socket when the pieces are new. However, the impact socket may, over a long period of use, become “damaged”, resulting in a looser fit to the square portion of the anvil. This increased clearance between the square portion interface and the impact socket allows the centerline of the square portion of the anvil and the centerline of the impact socket to become non-parallel. When this occurs, the theoretical line contact between the two that exists axially along the interface of the square portion and the impact socket becomes points of contact. These points of contact form at the sharp radii in the transition between the round body and the square drive and lead to zones of increased stress.
Moreover, as the impact socket becomes “damaged”, the corners of the impact socket tend to “dig” into the sharp radii in the transition. This digging between the impact socket and the square portion can damage the anvil, resulting in stress concentration zones. As the stress builds at these points, the anvil may fail at the stress concentration zones. This then can contribute to an early failure of the anvil.
One solution to the problem of sharp radii in an anvil is to increase the overall strength of the anvil. For example, increases in the amount of alloying elements such as carbon or nickel in the steel have been attempted. Unfortunately, this alloying leads to increases in the amount of retained austenite within the anvil. The retained austenite inhibits strength for impact loading and often leads to fatigue failures. Accordingly, there remains a need to provide an improved anvil design that reduces the stress concentration zones and prolongs the life of the anvil.